This invention relates to a polyamide-imide resin paste.
Usually, a resin solution as such does hardly show a thixotropic property. Thixotropy is defined as a phenomenon wherein an apparent viscosity is temporarily lowered due to deformation even in an isothermal state. This property is a fluidizing property indispensable for a screen printing paste in which a viscosity is required to be temporarily lowered at a high shear speed in printing to cause a fluidized state and further no sagging or running is required after transferred onto a material. One method for giving a thixotropic property to a resin solution is to disperse resin fine particles as a filler into a resin solution to form a paste. A wide variety of such pastes are known.
As the resin solution which is to be applied for use not requiring great resistance, there have been proposed resin solutions of, for example, a rosin-modified phenol resin, a rosin-modified maleic resin, a melamine resin, an epoxy resin and the like, while there are known, for the use requiring a high heat resistance, resin solutions of a polyamide resin which is a precursor of a polyimide resin, a solvent-soluble polyimide resin and the like. Also, as the resin fine particles which may be dispersed in such a resin solution to form a paste, there are known fine particles of an aliphatic polyamide resin, a melamine resin, an epoxy resin, a phenol resin and the like for the use not requiring heat resistance so much, and fine particles of a polyimide resin and the like for the use requiring a high heat resistance.
For uses requiring a high heat resistance such as a semiconductor chip, the surface of which is to be coated with a paste, there has been employed a paste in which polyimide resin fine particles are dispersed in the polyamide resin or polyimide resin solution as stated above. However, where a paste with the vehicle of a polyamide resin solution is employed, there is the disadvantage that a higher temperature and a longer period of time is necessary for curing in forming a surface-protective film on a semiconductor chip, which leads to a remarkable decrease in productivity. Furthermore, these materials tend to shrink when cured to break down wires. Moreover, where a paste with the vehicle of a polyimide resin solution is employed, the above-depicted problems could be solved, but there remains a problem in regard to cost, for application to a wide use, owing to a high expense of the polyimide resin.
In addition, such highly heat-resistant resins have been employed in the form of a solution in a highly polar nitrogen-containing solvent, e.g., N-methylpyrrolidone because of their poor solubility. However, the screen-printing paste prepared by employing a nitrogen-containing solvent has a disadvantage that the paste would adhere onto screen during printing to cause clogging and thus make printing impracticable because of a high hygroscopicity of the nitrogen-containing solvent. Besides, the paste has another disadvantage that it tends to swell or dissolve the emulsion constituting the screen-printing plate owing to a superior dissolving ability of the nitrogen-containing solvent, which results in a shortened life of the plate.